In the early twentieth century, Marcel Tolkowsky calculated his measurements for the ideal cut parameters in brilliant cut diamonds as a pavilion mains angle of 40.75°, a crown angle of 34.5°, and a 53% table, with a lower halves angle about 2 degrees steeper than the pavilion mains. In discussion of gemstone cuts, facet angles and dimensions are generally taken with respect to a table angle of 0° and the average diameter of the gemstone measured at the girdle (“diameter” or “width”). The proportions of round cut diamonds commonly preferred by twentieth century precision diamond cutters for super ideal cut diamonds are as follows:
TABLE ATypical Round Brilliant Proportions for Ideal CutsTable width (size):54-57%Crown angle:34-35°Pavilion angle:40.6-41°Total depth:60-62.5%Girdle thickness:Thin to mediumCulet:None (pointed pavilion)
These numbers remained fairly constant, within ranges, until around 2004, when the American Gem Society Laboratories (“AGSL”) developed the angular spectrum evaluation tool (ASET®) imaging disclosed in U.S. Pat. No. 7,355,683, hereby incorporated herein by reference, as a way to determine how a gemstone, such as a diamond, for example, is handling and returning light to the observer. ASET imaging can be used to generate light performance maps such as the ASET® maps by illuminating the gemstone with green, low angle light from the sides, in the upper hemisphere up to 45 degrees from the horizontal plane of the table facet, representing low-intensity light from an indirect source; red, medium angle light from above at 45 to 75 degrees from horizontal, representing brightness or the brightest, high-intensity light; and blue, high angle light from above at 75 to 90 degrees, representing areas of obstruction, i.e., light that the diamond cannot take in due to the body of the observer, which is useful in assessing contrast as seen by the observer. ASET maps are also useful to assess light leakage, which may appear as a black or gray area in the ASET maps or white if backlighting is used. Modern precision diamond cutters are fortunate to have a variety of tools at their disposal to plan how to cut a particular diamond from the rough, or re-cut a previously cut diamond, for symmetry and light performance. For example, the AGSL can mathematically calculate the light performance grade for a virtual diamond with a given set of proportions, e.g., the diamond cutter can confirm that the proportions proposed for cutting the gemstone should obtain a light performance grade of ideal 0, i.e., no deductions. As used herein, cut and light performance grades are determined according to the standards of the American Gem Society (“AGS”) and/or AGSL as of the filing date of this application. In the event of any conflict the AGS shall control. Using ASET imaging, as well as ray tracing and virtual diamond modeling software, the AGSL has determined the combinations over a range of table percentages, crown angles, and pavilion angles, that are needed to obtain ideal cut proportions for maximum light return or brilliance. The procedure is described in Yantzer, Peter et al., “Foundation, Research Results and Application of the New AGS Cut Grading System”, American Gem Society (2005), published at [[https://www.]] http://c.ymcdn.com/sites/www.americangemsociety.org/resource/resmgr/docs/AGSLab/AGS-Cut-System.pdf. The AGSL has made available cutting chart guidelines for a standard round brilliant cut at [http://www.] americangemsociety.org/page/roundguidelines, and the AGSL Proportion Charts (2008), published at [[https://www.]] agslab.com/docs/pbcg/AGSLProportionCharts.pdf. These charts are based on an 80% lower halves length, and do not take the angles of the lower halves into account, reflecting the general assumption in the industry that the lower halves are not at all a factor in maximizing light performance. In using these charts, however, the process for planning and executing the cutting of a diamond takes the final weight of the gemstone into account as a primary target, and thus the diamantaire seeking a high light performance may frequently select the steepest angles and largest depths possible at the margins of the values indicated in the charts.
Despite all of these modern tools, applicant has discovered the presence of low-intensity green hemisphere light is often seen in the peripheral areas of tables of diamonds cut using the AGSL charts, even with the crown and pavilion angles otherwise predicting or providing a light performance grade of ideal 0. This green in the peripheral table detracts from the achievement of maximum light performance as seen in the ASET maps. This peripheral green seen in the table periphery by ASET imaging is referred to herein as the “green table effect” or “green in the table.”
Table 1 has the proportions of virtual diamonds 1A and 1B discussed in Example 1 below. FIG. 1 illustrates a typical ASET image 10 for a round brilliant cut (non-inventive) diamond 1A having an AGS light performance grade of ideal 0 exhibiting the green table effect, contrasted with a similar cut for an inventive diamond cut according to an embodiment of the present disclosure shown in FIG. 2, and discussed in more detail below. The red regions 12A, 12B indicate light reflection or return from the red hemisphere, which are the brightest areas of the diamond, and most desirable. The green regions 14A, 14B indicate low level light return from the green hemisphere, and are less desirable. The green region 14A in the periphery of the table is the green table effect. The blue regions 16A, 16B indicate a contrast pattern of dark reflections indicative of the symmetry of the cut. The blue regions 16A, 16B form a familiar pattern of arrows seen in certain precision gemstone cuts with a high degree of symmetry—the corresponding image of the crown is a hearts pattern, both of which can be seen in a hearts and arrows viewer instrument. The black regions 18, which may also appear white on some ASET images with backlighting, represent areas of light loss, which are the most undesirable.
One exception to the red-green distinction noted above is that the center region or table reflection of the table facet may sometimes display green light or red light in the ASET image, depending on the angle of the pavilion mains. Above a pavilion mains angle of 40.768° with respect to the table plane, the table reflection is normally red; and below 40.768°, normally green. The appearance of green or red in the central table reflection does not appear to adversely impact the light performance grade of the gemstone. The green table effect as discussed herein does not concern the central table reflection.
The industry appears to be unaware of the green table effect, and at a loss to understand when the green table effect will occur, or how to avoid the green table effect, to obtain super maximum light performance and an ASET map without the peripheral green in the table. Diamantaires are thus in need of ways both to avoid the occurrence of this green table effect in a gemstone with an otherwise expected ideal cut and AGS light performance grade of ideal 0, and if seen in a cut stone, to polish or re-shape the stone to eliminate the green table effect.